'Allegiant' is the highly-anticipated conclusion to Veronica Roth's Divergent series.

by Lindsay Deutsch, USA TODAY

by Lindsay Deutsch, USA TODAY

Veronica Roth is one of the biggest names in the book world - and at 25, she's about to get even bigger. The Chicago-based author concludes her best-selling dystopian Divergent trilogy with Allegiant (Katherine Tegen Books, on sale Oct. 22), with droves of fans eager to see what becomes of Roth's heroine, Tris, and her love interest, Four. And the series is getting the Hollywood treatment, with a movie due out in March starring Shailene Woodley, Theo James and Kate Winslet. Roth spoke with USA TODAY's Lindsay Deutsch on her 25th birthday:

1. How did the idea for the Divergent trilogy come together, and now that you've finished, how has it evolved?

Originally it came from this concept in psychology called exposure therapy where people with phobias are repeatedly exposed to their fears until their brain reconditions it. That's the little nugget, and it took on a life of its own. As Tris grows up and her world gets bigger, I'm doing the same thing at the same time. When this began, I'd just graduated college and my world just kept getting bigger.

2. What are fans are dying to see in the final book?

So far, Tris and Four's relationship has been somewhat chaste and I think everyone is rooting for things to get a little more physically intimate, let's just put it that way. In Allegiant, the relationship definitely develops, but I won't spoil more.

3. You went to Northwestern and live in Chicago now. Why did you use your city as a dystopian setting?

There's something really fun about it because it's so clean compared to other cities. Imagining it all destroyed and wrecked was really interesting - not that I want to see it destroyed and wrecked! It also just kind of appeared out of nowhere. When I wrote the first draft of Divergent, there were just a lot of trains and this marsh and I just realized that it's already in Chicago. My favorite buildings, Sears (Willis)Tower and the Merchandise Mart, are both in the books. So are the Ferris wheel and the Bean.

4. Did you envision certain characters in the starring roles for the movie adaptation?

I don't really have a clear image of my characters as I'm writing - it's kind of like I'm squinting at it from a distance. They're just blurry people who do things. But when Shailene (Woodley) and Theo (James) were cast and I saw their screen test together, it was absolutely perfect.

5. What writers have inspired you?

I had this stretch of childhood where I read a ton of children's fiction, then I grew up and read it all over again. I'm a big fan of Lois Lowry, of The Giver, and Madeleine L'Engle, of A Wrinkle in Time. But my dad also instilled in me a science fiction streak. I read Dune by Frank Herbert when I was far too young to understand it.